1. Field of the Invention
The present invention relates to a process for producing a carbonaceous preform and particularly to a laminated carbon fiber reinforced carbonaceous composite preform, namely, a so-called carbon/carbon composite preform.
2. Prior Art
A conventional process for producing a carbon fiber reinforced carbonaceous composite preform comprises the steps of producing a reinforcing carbon fiber or a carbonaceous preform by adding a binder thereto and densifying the foregoing by means of a carbonaceous matrix.
The densification in the manufacturing procedure of the carbon fiber reinforced carbonaceous composite preform is carried out by any one of the three methods, namely, a method wherein a thermosetting resin is used, a method wherein a thermoplastic substance, such as a pitch, is used and a method wherein a CVD is utilized.
A carbon fiber reinforced carbonaceous composite preform, which is prepared by use of a thermosetting binder such as a phenolic resin, is associated with drawbacks that a delamination tends to occur because a contraction of volume takes place during carbonization and that the obtained composite preform is inferior in thermal conductivity and strength.
A carbon fiber reinforced carbonaceous composite preform, which is prepared by use of a pitch-based matrix in order to avoid the above-mentioned drawbacks, is associated with a drawback that a delamination tends to occur because a volume expansion takes place during carbonization due to foaming of the pitch-based matrix, which is not thermally curable, thereby presenting an impediment to the formability and mass production.
For example, JP4-76342B discloses a process for producing a carbonaceous preform by carbonizing a matrix, which utilizes a carbonaceous pitch as a starting material, and a carbon fiber under uniaxial pressing.
Although this process provides a carbonaceous preform excellent in such formability as flatness and dimensional accuracy, the remaining problem has been that the foaming of pitch during the carbonizing treatment seriously contaminates the inside of the carbonizing oven to an extent that the maintenance work is so often required that a mass production becomes impossible. In addition, even if the carbonizing condition is optimized to suppress the foaming of pitch to a minimum, it has been impossible to completely prevent the delamination of the obtained preform.
Further, Hot Isostatic Pressing (HIP), as a method for solving the problem of gas generation from the pitch during carbonization, is described, for example, in JP5-4945B.
Although the present inventors have carried out the HIP treatment in accordance with the above-mentioned description by placing carbon fiber fabrics and pitch in a vessel for the treatment, a satisfactory preform could not be obtained because the insufficient bonding between the fabrics caused insufficient formability and poor shape retainability.
Furthermore, JP3-52426B discloses the preparation of a carbon fiber reinforced carbonaceous composite preform by a process comprising the steps of preforming a carbonaceous material, which consists of a carbon fiber, a pitch matrix and a carbonaceous powder, by a complicated treatment, fixing the obtained preform by a fixing means comprising stainless steel plates and densifying the preform, characterized in that the densification does not cause expansion and delamination.
However, the drawbacks of this process are that the manufacturing procedure is complicated because the procedure consists essentially of laminating a matrix powder and a carbon fiber fabric alternately, uniaxially pressing, fixing by a fixing means and carbonizing the laminate and that the delamination in the preparation of the carbonaceous preform cannot be completely prevented.
Meanwhile, according to JP5-306180A or JP55-51769A, a carbon fiber reinforced carbonaceous composite preform can be obtained by a process comprising the steps of press-fixing a carbon fiber felt to a desired thickness or shape by use of jigs made from graphite or ceramics and placing the thus-fixed felt in a vacuum oven to carry out a CVD treatment, characterized in that the carbon fiber reinforced carbonaceous composite preform can be obtained by a process which does not use a conventional binder or hot pressing.
However, the problem of the CVD process is, for example, that the procedural steps are complicated and the strength of the carbon fiber is caused to diminish, because a long time period is generally required for the densifying treatment although such phenomena as foaming and expansion of pitch are eliminated, because frequent surface treatments need to be performed during the densification in order to minimize the formation of closed voids as the densification proceeds and because a surface treatment of a carbon fiber with sand paper or the like is necessary at the time of laminating operation.